1. Field of the Invention
The present invention relates to a circuit for and a method of driving a current-driven device such as an organic EL (electroluminescent) element, and more particularly to an image display apparatus which incorporates such driving circuits and employs current-driven devices as light-emitting elements.
2. Description of the Related Art
In recent years, attention has been attracted to image display apparatus which employ current-driven light-emitting devices such as organic EL elements, for use in computer output devices and cellular phones. The organic EL elements, which are also referred to as organic light-emitting diodes, are advantageous in that they can be driven with a direct current (dc). If organic EL elements are used in an image display apparatus, then they are generally arranged as respective pixels in a matrix on a substrate, providing a display panel. Attempts have been made to construct the image display apparatus in an active matrix configuration in which the organic EL elements of the respective pixels are driven by TFTS (thin-film transistors), which have a MOS (metal oxide semiconductor) transistor structure, formed on the substrate.
Since the organic EL elements are current-driven devices, if they are driven by TFTs in an image display apparatus, then the image display apparatus cannot use the same circuit arrangement as an active matrix liquid crystal image display apparatus which employs liquid crystal cells that are voltage-driven devices. Heretofore, there has been proposed an active matrix drive circuit having organic EL elements and TFTs connected in series with each other and inserted between a power supply line and a ground line, with control voltage being applicable to the gates of the TFTs, holding capacitors connected to the gates of the TFTs for holding the control voltage, and switch elements disposed between a signal line for applying the control voltage to the pixels and the TFTs. In the proposed active matrix drive circuit, the control voltage for the pixels is outputted on the signal line in a time-division multiplexed manner, and the switch elements are controlled so as to be rendered conductive only when the control voltage is outputted to the corresponding pixels. As a result, when a switch element is rendered conductive, the control voltage is applied to the gate of the corresponding TFT, causing a current depending on the control voltage to flow through the organic EL element and charging the holding capacitor with the control voltage. When the switch element is then rendered nonconductive, the holding capacitor keeps on applying the control voltage to the gate of the TFT, continuously causing the current depending on the control voltage to flow through the organic EL element.
Gazette WO 99/65011 discloses a drive circuit having the above circuit arrangement which is suitable for driving current-driven devices such as organic EL elements. FIG. 1 shows the drive circuit disclosed in WO 99/65011. Though n-channel MOS FETs (field effect transistors) are used as drive transistors for driving the current-driven devices (organic EL elements) in a common cathode configuration in WO 99/65011, p-channel MOS FETs are used as drive transistors for driving the current-driven devices in a common anode configuration in FIG. 1.
The drive circuit shown in FIG. 1 has power supply line 1 and ground line 2, and drive transistor 7 as a p-channel MOS FET having a source connected to power supply line 1. Holding capacitor 6 is connected between power supply line 1 and the gate of drive transistor 7, which is connected to one end of a switch element 9. The drain of drive transistor 7 is connected to the other end of the switch element 9 and an end of switch element 10 whose other end is connected to the anode of current-driven device 11. The cathode of current-driven device 11 is connected to ground line 2. A current flowing from drive transistor 7 into current-driven device 11, i.e., a drive current, is represented by Idrv.
Signal line 3 is provided in order to indicate the drive current Idrv to flow into current-driven device 11. Signal line 3 is connected to an end of switch element 8 whose other end is connected to the drain of drive transistor 7. A current flowing through signal line 3 is represented by Iin.
Switch elements 8 through 10 are turned on and off depending on external control signals, and comprise MOS FETS, for example. Control signals for switch elements 8 through 10 are generated by a control signal generating circuit, not shown, and supplied from output terminals of the control signal generating circuit through control lines, not shown, to switch elements 8 through 10. If switch elements 8 through 10 comprise MOS FETS, then control signals therefor are binary signals electrically representing a ground potential or a power supply potential, and applied to the gates of the MOS FETS.
The drive circuit shown in FIG. 1 is a circuit for driving one pixel, i.e., one current-driven device 11. If an image display apparatus comprises organic EL elements used as current-driven devices 11, then current-driven devices 11 are arranged in a matrix as described above, and the drive circuit shown in FIG. 1, particularly a circuit enclosed by the broken line, is associated with each of current-driven devices 11. Power supply line 1 and ground line 2 are provided commonly for each drive circuit, and signal line 3 is provided commonly for each vertical array of drive circuits, i.e., a column of drive circuits. The control lines are provided commonly for each horizontal array of drive circuits, i.e., a row of drive circuits.
The current-driven devices and the drive circuits thus arranged in a matrix make up an active matrix image display apparatus. Because of structural features of the drive circuits and the image display apparatus, each signal line 3 extends across the control lines for controlling switch elements 8 through 10 and power supply lines 1 and ground lines 2, with an insulating layer interposed therebetween, and parasitic capacitors are produced in regions at the points of intersection where signal line 3 traverses the control lines, power supply lines 1, and ground lines 2. If the current-driven devices 11 comprise organic EL elements, then the regions where the cathodes of current-driven devices 11 connected to ground line 2 cross signal lines 3 have a large area, and parasitic capacitors produced in those regions are not negligible. As a result, as shown in FIG. 1, an equivalent parasitic capacitor is formed between signal line 3 and power supply line 1, and another equivalent parasitic capacitor is formed between signal line 3 and ground line 2. The capacitance of each of these equivalent parasitic capacitors depend on the number of pixels and the structure of the image display apparatus, and may, for example, be at least 10 times the capacitance of holding capacitor 6 at each pixel.
Operation of the conventional drive circuit shown in FIG. 1 will be described below. It is assumed for the description of operation that a number of current-driven devices 11 are arranged in a matrix and combined with respective drive circuits.
The control signal generating circuit generates control signals for successively selecting rows of drive circuits one at a time, and supplies the control signals to switch elements 8 through 10 of the drive circuits through the control lines. In synchronism with the control signals, a signal current Iin is supplied to signal lines 3 for the drive circuits belonging to the selected rows. As a result, the signal current Iin flows into drive transistors 7 of the drive circuits in the selected rows, and corresponding holding capacitors 6 hold a potential depending on signal current Iin. When those drive circuits are unselected because the control signals select a next row of drive circuits, the drive circuits keep driving respective current-driven devices 11 with the same drive current Idrv as the signal current Iin.
FIG. 2 shows a timing chart of operation of the drive circuits. First, details of operation of the drive circuits in a selected period will be described below.
When a certain row of drive circuits enters a selected period, switch elements 8, 9 are rendered conductive (i.e., ON state) and switch element 10 is rendered nonconductive (i.e., OFF state). A certain shorter period in the leading end of the selected period serves a reset period, and during the reset period, the potential of signal line 3 is preferably held at the power supply potential, and the potential of signal line 3 and the potential of drive transistor 7 are preferably reset to the power supply potential. After elapse of the reset period, a signal current Iin which is equal to a current to flow into current-driven device 11 is supplied to signal line 3. The signal current Iin may be supplied to signal line 3 during the reset period.
In the illustrated example, the signal current Iin represents the sum of a drain current flowing from the drain of drive transistor 7 toward signal line 3, a current flowing to charge parasitic capacitor 4 and holding capacitor 6, and a current discharged from parasitic capacitor 5. When the reset period is over and the signal current Iin starts to flow, the signal current Iin charges parasitic capacitor 4 and holding capacitor 6, parasitic capacitor 5 is discharged, and the gate potential of drive transistor 7 is gradually lowered until finally a gate-to-source potential corresponding to a drain current equal to the signal current Iin is developed on drive transistor 7.
If the signal current Iin is sufficiently large, then since parasitic capacitor 4 and holding capacitor 6 are charged and parasitic capacitor 5 is discharged quickly, the drain current from drive transistor 7 reaches the signal current Iin during the selected period, and the voltage across holding capacitor 6 reaches a value to produce a drain current equal to the signal current Iin. If the signal current Iin is small, then the charging of parasitic capacitor 4 and holding capacitor 6 and the discharging of parasitic capacitor 5 are not completed during the selected period. Therefore, the drain current from drive transistor 7 does not reach the signal current Iin, and the gate-to-source potential of drive transistor 7 does not reach a value corresponding to a drain current equal to the signal current Iin.
When the selected period is over and an unselected period is reached, switch elements 8, 9 are rendered nonconductive and switch element 10 is rendered conductive at the start of the unselected period. As a result, drive transistor 7 supplies the drive current Idrv to current-driven device 11. As the gate of drive transistor 7 is disconnected from signal line 3, the gate potential of drive transistor 7 is held at a value determined immediately before the unselected period is reached, by the action of holding capacitor 6. If the signal current Iin in the selected period has been sufficiently large, then since the gate potential of drive transistor 6 has been determined at a value corresponding to a drain current equal to the signal current Iin, a drive current Idrv equal to the signal current Iin continuously flows into current-driven device 11. Therefore, the relationship Iin=Idrv is satisfied. Conversely, if the signal current Iin in the selected period has been small, then since the gate potential of drive transistor 6 has not reached a value to supply a drain current equal to the signal current Iin, a drive current Idrv different from the signal current Iin continuously flows into current-driven device 11. Therefore, the relationship Iin≈Idrv is satisfied.
FIG. 3 shows the relationship between the signal current (input signal) Iin and the drive current Idrv in the drive circuit shown in FIG. 1. If current-driven devices 11 comprise organic EL elements, then the graph represents the relationship between signal current Iin that is input and the luminance. In FIG. 3, an ideal relationship is indicated by the broken-line curve, and an actual relationship between the signal current and the drive current by the solid-line curve. It can be seen from FIG. 3 that the conventional drive circuit is unable to provide a drive current corresponding to signal current Iin in a region where signal current Iin is small.
As described above, the conventional drive circuit cannot provide a desired drive current when the input signal (signal current) is small because of the times required to charge and discharge the parasitic capacitors and the holding capacitor. If the conventional drive circuit is incorporated in an image display apparatus, then the image display apparatus fails to provide a desired level of luminance. Particularly if the conventional drive circuit is incorporated in an image display apparatus using organic EL elements, then because a current flowing into an organic EL element corresponding to each pixel is very small, images displayed by the image display apparatus are liable to deteriorate, and the luminance controllability thereof is lowered.